Add stats to serial CLI

[michaelhart]

Implemented formatStatsReply, formatRadioStatsReply, and formatPacketStatsReply methods in MyMesh for both simple_repeater and simple_room_server. Introduced StatsFormatHelper for structured statistics formatting. Updated CommonCLI to handle new stats commands.

The 3 new serial-only commands are stats, stats-radio, and stats-packets.

  -> {"battery_mv":0,"uptime_secs":17,"errors":0,"queue_len":2}
  -> {"noise_floor":-103,"last_rssi":-17,"last_snr":12.25,"tx_air_secs":1,"rx_air_secs":3}
  -> {"recv":6,"sent":3,"flood_tx":3,"direct_tx":0,"flood_rx":6,"direct_rx":0}